Navigate to the desired screen, press and hold the power and volume down buttons at the same time for a screen capture. It worked for me but took a couple of tries since the volume and power buttons are around back.

Also you can use an app to take a screenshot on an unrooted LG G Flex:

Install Xposed framework- gravity box. This app allows you to create a customized key binding for the selected command. Then enable screenshot option on power menu or whatever buttons you want now.

If you are rooted, there are a number of Apps in the Google Play Store that will allow to take screenshots of your phone’s screen.